06 May IDENTIFY THE NAME(S) OF YOUR STREAM(S) SUB CATCHMENT(S).
Topic: a mapping exercise delineating catchments
The aim of this tutorial exercise is to become acquainted with the process of delineating catchments manually and to consider
drainage patterns within catchments. Topographical maps upon which students can trace catchment boundaries should be obtained
(desirably maps of local areas of interest) either by downloading appropriate maps from a website (see below), purchasing an
appropriate map or using maps in your place of work or in a library.
Methods:
1) If available use 1:250,000 topographic maps to choose 2 small subcatchments from within a relevant catchment of interest.
Ideally, the subcatchments should have different features such as topography or rainfall or show different land use e.g.
forested versus agriculture, cropland versus pasture, agriculture versus urban.
2) Identify the name(s) of your stream(s) sub catchment(s).
3) Use 1:250,000 maps to roughly define the subcatchments. Then use
1:100,000 topographic maps to more accurately define your subcatchments by marking the watershed (basin or catchment
boundary) directly on the map
if laminated or on tracing paper. The method of delineating catchments will
follow that stated by Gordon et al.pdf, wherein: “boundaries are drawn by following the ridge tops which appear on
topographical maps as downhill-pointing V-shaped crenulations. The boundary should be perpendicular to the contour lines it
intersects. The tops of mountains, often marked as dots on a map, and the location of roads which follow ridges are other
clues.” Some “extrapolation” may be needed in subcatchments with low topographic relief by drawing lines approximately half
way between streams flowing in opposite directions.
4) Using the contour lines on the 1:100,000 maps, describe the topography of your subcatchments (are they flat or hilly?).
Are there differences in topography between your subcatchments?
5) Compare the stream drainage system (network) of your subcatchments.
Determine the type of drainage pattern using Figure 4.9 in Gordon et al.pdf. If many intermittent stream channels are shown
on your map the subcatchment either has low rainfall or very porous soils. The drainage network is a geohydraulic continuum
between surface waters and groundwater. Water flowing at the surface at one place may be underground at another; streams may
cease to flow at one point but recommence to flow downstream. Interaction zones between surface water and groundwater are
important attributes of the landscape. You may be able to find information on depth to groundwater in your subcatchment which
would indicate near surface groundwater and thus potential areas of recharge of
groundwater by surface waters or of discharge of groundwater to surface waters.
6) Determine the area of your subcatchments using either the “counting squares” or the weight method (see Gordon et al.pdf).
Trace your subcatchments onto a piece of graph paper and count the total number of squares (1cm x 1cm). This need only be
approximate. Using the same scale as on your map trace out a square of known area e.g. 10 km x 10 km (this may vary depending
upon the size of your subcatchments). Count the
number of squares in your known area and then divide the number of squares
for each subcatchment by this number. Alternatively, trace your subcatchments onto another piece of tracing paper and cut out
the subcatchments. Using the same scale as on your map, trace out a square of known area e.g. 10 km x 10 km (this may vary
depending upon the size of your subcatchments). Cut out this known area and weigh it. Measure the weight of your
subcatchments. Calculate the area of your subcatchments using the formula in Gordon et al.pdf.
Materials:
– 1:250,000 maps if available.
– 1:100,000 maps (laminated if possible).
– Tracing paper.
– Pencil.
– Eraser.
– Felt tip pen.
– Scissors.
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